An apparatus for combinatorial RF biasing at selectable spots includes one or more RF biasing elements that can be moved to or selectively activated to provide an RF hot spot. In one aspect, the invention is an externally powered, light-activated, sub-retinal prosthesis in which natural light entering the eye conveys visual details to the sub-retinal prosthesis, while wireless radiofrequency transmission provides the power needed to stimulate the retina, which would be insufficient if it were obtained from the intensity of incoming light alone. for treating a subject for prostate cancer. Daughter boards may be removed or installed on the main communication board while the main communication board is receiving power. A second mask, including apertures alignable over the pixel regions of the field emission array, is disposed over the passivation layer. The apparatus also includes a pair of fiber retainer rails interposed between the fiber cassette and the fiber take up mandrels for guiding the fiber optic cable from the fiber cassette to the fiber take up mandrels.